This invention relates to applications of accurate measurement of geographical location including elevation as for example obtained by GPS information for use in control and more particularly in terrestrial spectral matching for control of infrastructure artificial lighting systems. This invention involves a mathematically rigorous proof of validity of the conclusions underlying embodiments of the invention. The present application contains a tutorial in support of the technology underlying the invention.
Many events and functions in daily life are controlled by natural light, and in particular by the sun's elevation angle. An example can be found in the field of agriculture and artificial lighting applications. In other words, lights for artificial illumination are turned on when it gets “dark.” Many agricultural activities are done during the “day” or at “night.” Darkness is thus a subjective term; it depends on the light sensitivity of the human eye in the visible spectrum. When the intensity of the sunlight goes below a certain level, which the human eye can't clearly see, this condition corresponds to “dark.” When it is “dark” the need of artificial illumination is required to see comfortably. Since the sun is the major source of light on earth, received sunlight at a location on earth is directly related to the elevation angle of the sun at that location at that time from the horizon. The elevation angle of the sun at a location is a function of the latitude, longitude and altitude along with the date and time. When the sun is below the horizon, even it is not directly visible, there is enough light where the human eye can see comfortably. This is due to the scattered sunlight from the atmosphere, a phenomenon explained herein below.
There are basically three types of artificial illumination controls in use today. They are based on 1) the local time with a timer, 2) light intensity sensing circuits and 3) a combination of the first two. Each has particular disadvantages, as herein after explained.
Timer Based Control
The simple timer circuit control of the prior art involves setting the timer to local time so that light switches on and off at programmed times. However, the sunrise and sunset times vary a great deal over the year anywhere. Therefore for every location on earth the sunrise and sunset times based on local time needs to be calculated and supplied as a table. A general table can't be very accurate as well because the sunrise and sunset times are also altitude dependent. If the timer is not set correctly for that location for every day, some days the lights will turn on when there is more than enough light, wasting energy and some days the lights will be activated below the comfortable light intensity levels, causing issues with visibility. Once it is programmed for the dates and location, the timer will work as intended only at that location. A general factory preset is useless because of such factors. There are also issues related to time adjustments needed for the daylight savings time, for which there is no set standard, and related to the precision and accuracy of the clock.
In some places, astronomical routines are used to control lighting. These are considered timer based controls. They are typically set to sunrise and sunset times, not to a time corresponding to any sun elevation angle chosen. Time settings can only be roughly hard coded approximately corresponding to 0 to −0.5° of sun elevation angles. There are no known astronomical controls available that are customizable to any sun elevation angle nor specifically to the preferred angle as disclosed in this application, and which also takes the altitude and topography related effects into consideration.
Light Intensity Based Control
The second type of controller uses a photocell light detector preset to a light intensity level These types are known commercially as dusk-to-dawn detectors. They are cheap and most of the intelligent light controls today for outdoor lighting employ this method. Compared to the first type, they appear superior, but they have their own problems when the threshold level of light intensity is low for this application.
Off-the-shelf dusk-to-dawn detectors have a great deal of variation on light detection levels depending on the orientation, placement, temperature, and manufacturing differences but it is conclusively seen that they can't detect light intensity levels consistently when sun the elevation angles are lower than 1.5 degrees below the sunrise and sunset sun elevation angles for the same location. The photocell light sensitivity variation is very wide, even from the same manufacturer, and it is not consistent throughout the year, being greatly influenced by the detector temperature. The practical variations in the sensitivity of the dusk-to-dawn detectors are found to be in the light intensity levels corresponding to 3 to −1.5 degrees of sun elevation angle. There are many reasons for this, primarily due to the physics related to the operation of the photocells or any type of photo detectors [21,23,24], so one shouldn't expect significant consistency and improvement in the sensitivity in the future without very costly improvements.
The physics of the method is based on measuring the sun's irradiance using the current generated by the photo electric devices such as photo cells, photo diodes, photo transistors or solar cells. Basically there is a photo sensor that converts the light into electrical current. The magnitude of this current relates to the received light intensity, wavelength and the band gap of the semiconductor used. If the sun's irradiance goes below a threshold level, the photo current will also goes below a threshold level. This condition is detected by a comparator and followed by a simple logic circuit that drives the switch of the light circuit to an “on” or “off” state. The switch can be electro mechanical such as a relay or a solid-state triggered device-thyristor, triac or an IGFET. As can be seen, it is an “indirect” method of determining conditions when the lights must be turned on.
There are some physical issues with this methodology:
i) The magnitude of the generated photo current in a semiconductor photo electric device of any kind is directly related to the photon flux received by it and the band gap of the semiconductor detector [21,23,24]. Photon flux at a given wavelength is the number of photons per second per area of the incoming radiation. Designing an accurate light sensor for medium to high level of light intensity where the sun is at high elevation angles is easy. Designing a light sensor that can accurately detect low level of light intensities corresponding to low sun elevation angles such as −3° to −4° of the sun is challenging. There are two physical issues for this difficulty in outdoor applications.
The first obvious reason is simply that the received photon flux at the light sensor is too low for certain targeted sun elevation angles for accurate and consistent detection. A good practical example is the fact that very few CCD still cameras can take pictures without a flash, whereas humans do not need any kind of artificial illumination!
The second reason is related to the spectral power density of the scattered light in the atmosphere for sun elevation angles in the range of −3° to −4°. As predicted by the ASAR program methods and visually observed, the scattered light spectral power spectrum shifts to dark blue to violet at these sun elevation angles as in the Scotopic luminosity function. Human eye wavelength sensitivity closely adapts to the change in the scattered light spectrum at dusk conditions. This is a very remarkable feature of the human eye. On the other hand the highest ultimate efficiency of silicon photo detector is very close to 1,000 nm, determined by its 1.12 eV band gap value and the Planck constant h. For a monochromatic radiation at 507 nm wavelength, as in the peak Scotopic Luminosity response of the human eye, its ultimate efficiency drops approximately to half of its efficiency compared to the 1,000 nm radiation. A wide band gap detector is need that has its band gap in the order of 2 eV, for detecting the dusk conditions with the same ultimate efficiency of Silicon at 1,000 nm radiation.
ii) The “dark” current operations of a semiconductor device used in sensing the light and its sense and compare circuits are very strong functions of temperature [21,23,24]. Since the light sensor has to be exposed to the outdoor light, it is also exposed to the outside temperature. The design specification requires accurate operation over a very wide range of temperature variations throughout the year, around the world. This makes the design of these circuits very difficult even for low level lighting conditions which occur when sun elevation angles go below 1° below the horizon. In other words, an accurate light sensor design for low level light sensing applications is itself a challenging circuit design. The resulting inaccuracies due to the temperature effects will affect the “on/off” times.
iii) The sensors must all be exposed to sunlight, which makes them very susceptible to dirt, dust, rain, water, frost and snow. This also creates a reliability and product life problem due to harsh environmental effects. Since the sensing is done for low light intensities, these effects become more important and difficult to eliminate.
iv) The optical to electrical conversion performance degradation of the light sensors over time can be in the order of 5-10% over a period of 5 years under the direct sunlight!
v) Since the sensors cannot detect low enough light intensities, the scattered light for sun elevation angles corresponding to at 1° to −1.5° still have some directional dependency, which means that any object will still generate a shadow that gives them a placement and orientation dependency in their “on/off” times. The sunrise and sunset azimuth changes greatly for any location over the year and this will cause the orientation and the length of an object's shadow to be a complex variable of date and time. If the detector is in the shadow of any obstruction such as vegetation, landscape or buildings for that particular date and not for any other date, the detector will trigger at different times.
In practice all these issues are not eliminated, just avoided, by setting the threshold current in the sensor to higher levels than needed. This causes the switch to operate no better than sunlight intensity levels corresponding to 1°-1.5° below the horizon, where visibility is good for the majority of the population. This causes significant waste of electrical energy.
Study also included several types, even the “first class thermopile” pyranometers that are widely used in solar energy applications. None had sensitivity to detect light intensity levels corresponding to 1 degrees of sun elevation angle above the sunrise and sunset for the same location where they are installed. This was also confirmed with the discussions made with their manufacturers. Although they are far more accurate and consistent compared to simple and cheap dusk-to-dawn detectors for higher sun elevation angles, expecting performance well for this application is not realistic and is not even recommended by their manufacturers given the physics behind their operation principles.
As a final example consider the lower range of brightness measuring capability of well-calibrated light meters that are used in professional photography, which are in the order of 1 cd/m2. However, they are only calibrated with photopic luminosity function. At the sun elevation angles of present interest, the light intensity is in the Mesopic range, so the measurement values that they provide are not valid for human eye vision for these conditions.
Hybrid Techniques
Since low level light detection is difficult, this is done with the timer, and any other loss of light intensity due to weather effects is done by light intensity sensing sensors. The disadvantages and advantages of both methods remain and thus the hybrid technique does not resolve the adjustment-related issues of the timer.
Thus “cheap” and “reliable” scattered light intensity detection hardware corresponding to −3° to −4° of sun elevation angle is not known or available in the market today. To maintain an acceptable and consistent comfort level in outdoor lighting, the light detection sensitivity of any light intensity sensing device available today has to be set to light intensity levels corresponding to not less than 1.5° of sun elevation angle below the horizon. As can be seen, the light sensitivity levels achievable using with widely used standard measurement methods in use today do not provide the goal of the outdoor light intensity levels corresponding to the sun elevation angle of 3° to 4° below the sunrise and sunset.
What is needed is to find out how much energy savings can be done, if by any other means the artificial illumination can be activated only when the sun elevation angle becomes lower than 3° to 4° below the sunrise and sunset. As can be seen the answer to this question is not easy and can't be given without some work. The best way of finding this out is using a very accurate simulator which can predict the sun's location, its elevation and azimuth, at any date, any altitude and any geographic location on the world and compare it through simulations having the sun elevation angle as a parameter [7]. If the energy savings becomes significant by having the capability of employing a technique which can consistently and accurately predict 3° to 4° of sun elevation angle below the sunrise and sunset anywhere, any date and any inhabited altitude, then it will be worthwhile to work on an improvement.
Thus, what is needed is a mechanism to take advantage of the scattered light phenomenon and to automate response to greater negative elevation angles of the sun at any arbitrary terrestrial location and elevation.